Pain tolerance varies widely among peoples and, depending on external and internal circumstances, varies within an individual from moment to moment. Although the concept of pain tolerance is familiar, the neurobiological substrates that regulate pain tolerance are unknown. Pain tolerance requires motivational, emotional, and cognitive processing in addition to the sensory dimension of the pain experience. Achieving an understanding the specific neurons and circuits in the brain that regulate pain tolerance may to lead to breakthroughs that will enhance the capacity of patients to cope with intractable pain. This project utilizes innovative techniques to both assess pain tolerance behavior in animals, and to dissect the neural circuits thought to regulate pain tolerance. We have discovered that a subtype of pyramidal neuron in limbic cortical areas including the anterior cingulate cortex and insula becomes hyperexcitable during periods of lowered pain tolerance produced by injury. These neurons express GRM2, the gene encoding group II metabotropic glutamate receptors, which is a potential molecular target to control pain tolerance in clinical settings. The goals of this project are: 1) To identify the thalamo-limbic pathways that regulate pain tolerance; 2) To determine the role of GRM2 limbic cortical neurons in pain tolerance and nociceptive-withdrawal behaviors, and, 3) to determine whether pain tolerance can be modulated by pharmacological manipulation of group II metabotropic glutamate receptors. Optogenetic and pharmacological experiments will be performed in vivo in mouse to learn whether pain tolerance can be modulated independently of standard nociceptive-withdrawal thresholds. Modulation of neural activity in models of inflammatory and neuropathic pain will be tested to determine whether injury-induced changes to pain tolerance can be reversed. Neuro-anatomical and neurophysiological approaches in vitro will be used to generate new information on the plasticity of the thalamo-cortical circuits hypothesized to regulate pain tolerance and the membrane biophysics of GRM2 neurons. These experiments will be performed using animal models of persistent pain to determine whether function and anatomy of thalamo-cortical synapses and GRM2 neurons are altered by pain to provide mechanistic understanding for behavioral changes in pain tolerance. Pharmacological modulation of group II mGluR signaling will be tested in vitro to determine the potential for these receptors to be used in future clinical interventions. Completion of this project will generate new knowledge on the neural systems involved in the supraspinal processing of pain, including pain tolerance, and potentially catalyze an innovative shift in strategy for pain relief to enhance coping ability in chronic pain patients through neuromodulation.